Just after the sarin poison gas attacks on rebel-held districts of Damascus in August last year, I appeared on an American television programme with Razan Zaitouneh, a human rights lawyer and founder of the Violations Documentation Centre, who was speaking via Skype from the opposition stronghold of Douma in East Damascus.

She gave a compelling, passionate, wholly believable account of what had happened. “I have never seen so much death in my whole life,” she said, describing people breaking down the doors of houses to find everybody inside dead. Doctors in the few medical centres wept as they vainly tried to treat gas victims with the few medicines they had. Bodies were being tipped, 15 to 20 at a time, into mass graves. She contemptuously dismissed any idea that the rebels might be behind the use of sarin, asking: “Do you think we are crazy people that we would kill our own children?”

Ms Zaitouneh, 36, had been defending political prisoners for a dozen years and was the sort of credible advocate that won the Syrian opposition so much international support in its first years. But on 8 December, gunmen burst into her office in Douma and kidnapped her, along with her husband, Wael Hamada, and two civil rights activists, Samira al-Khalili, a lawyer, and Nazem al-Hamadi, a poet. None of the four has been heard from since. The group suspected of being behind the kidnapping is the Saudi-backed Army of Islam, although it denies being involved. Ms al-Khalili’s husband, Yassin al-Hajj Saleh, told the online publication al-Monitor: “Razan and Samira were part of a national inclusive secular movement and this led them to collide with the Islamist factions, who are inclined towards despotism.”

The kidnapping and disappearance of Ms Zaitouneh and the others have many parallels elsewhere in Syria, where Islamists have killed civil activists or forced them to flee. Usually, this has happened when the activists have criticised them for killings, torture, imprisonment or other crime. All revolutions have notoriously devoured their earliest and most humane advocates, but few have done so with the speed and ferocity of Syria’s.

Instead of modernising Syrian society in a progressive and democratic manner, the Salafi-jihadists want a return to the norms of early Islam and are prepared to fight a holy war to achieve this.

Why has the Syrian uprising, whose early supporters demanded that tyranny should be replaced by a secular, non-sectarian, law-bound and democratic state, so totally failed to achieve these aims? Syria has descended into a nightmarish sectarian civil war as the government bombs its own cities as if they were enemy territory and the armed opposition is dominated by Salafi-jihadist fighters who slaughter Alawites and Christians simply because of their religion. Syrians have to choose between a violent dictatorship in which power is monopolised by the presidency and brutish security services, and an opposition that shoots children in the face for minor blasphemy and sends pictures of decapitated soldiers to their parents.

Syria is now like Lebanon during the 15-year-long civil war between 1975 and 1990. I was recently in Homs, once a city known for its vibrant diversity but now full of “ghost neighbourhoods” where all the buildings are abandoned, smashed by shellfire or bombs. Walls still standing are so full of small holes from machine-gun fire that they look as if giant woodworms have been eating into the concrete.

Syria is a land of checkpoints, blockades and sieges, in conducting which the government seals off, bombards but does not storm rebel-held enclaves unless they control important supply routes. This strategy is working but at a snail’s pace, and it will leave much of Syria in ruins.

Aleppo, once the largest city in the country, is mostly depopulated. Government forces are advancing but are overstretched and cannot reconquer northern and eastern Syria unless Turkey shuts its 500-mile-long border. Government success strengthens the jihadists because they have a hard core of fighters who will never surrender. So, as the Syrian army advances behind a barrage of barrel bombs in Aleppo, its troops are mostly fighting the official al-Qa’ida affiliate Jabhat al-Nusra and the Salafist Ahrar al-Sham, backed by Qatar and Turkey.

The degenerate state of the Syrian revolution stems from the country’s deep political, religious and economic divisions before 2011 and the way in which these have since been exploited and exacerbated by foreign intervention. The first protests happened when they did because of the uprisings in Tunisia, Egypt, Libya, Yemen and Bahrain. They spread so rapidly because of over-reaction by state security forces firing on peaceful demonstrators, thereby enraging whole communities and provoking armed resistance. The government insists that protests were not as peaceful as they looked and from an early stage their forces came under armed attack. There is some truth in this, but if the opposition’s aim was to trap the government into a counter-productive punitive response, it succeeded beyond its dreams.

Syria was always a less coherent society than it looked to outside observers, and its divisions were not just along religious lines. In July 2011, the Brussels-based International Crisis Group (ICG) wrote in a report: “The Syrian authorities claim they are fighting a foreign-sponsored, Islamist conspiracy, when for most part they have been waging war against their original social constituency. When it first came to power, the Assad regime embodied the neglected countryside, its peasants and exploited underclass. Today’s ruling elite has forgotten its roots.”

In the four years of drought before 2011, the United Nations noted that up to three million Syrian farmers had been pushed into “extreme poverty” and fled the countryside to squat in shanty towns on the outskirts of the cities. Middle-class salaries could not keep up with inflation. Cheap imports, often from Turkey, forced small manufacturers out of business and helped to pauperise the urban working class. The state was in contact with whole areas of life in Syria solely through corrupt and predatory security services. The ICG conceded that there was “an Islamist undercurrent to the uprising” but it was not the main motivation for the peaceful protests that were mutating into military conflict.

Compare this analysis of the situation in the summer of 2011 with that two and a half years later. By late 2013, the war was stalemated and the armed opposition was dominated by the Islamic State in Iraq and the Levant (Isis), the former official al-Qa’ida affiliate now displaced by Jabhat al-Nusra.

Ideologically, there was not much difference between them and Ahrar al-Sham or the Army of Islam, which also seeks a theocratic Sunni state under Sharia law. Pilloried in the West for their sectarian ferocity, these jihadists were often welcomed by local people for restoring law and order after the looting and banditry of the Western-backed Free Syrian Army (FSA), the loose umbrella group to which at one time 1,200 rebel bands owed nominal allegiance. In Afghanistan in the 1990s the iron rule of the Taliban had at first been welcomed by many for the same reason.

The degree to which the armed opposition at the end of 2013 was under the thumb of foreign backers is well illustrated by the confessions of Saddam al-Jamal, a brigade leader in the Ahfad al-Rasoul Brigade and the former FSA commander in eastern Syria.

A fascinating interview with Jamal, conducted by Isis and translated by the Brown Moses blog, was recorded after he had defected to Isis and appears to be reliable, ignoring his self-serving denunciations of the un-Islamic actions of his former FSA associates. He speaks as if it was matter of course that his own group, al-Ahfad, was funded by one or other of the Gulf monarchies: “At the beginning of the Syrian revolution, the file was handled by Qatar. After a while, they switched to Saudi Arabia.”

Jamal says meetings of the FSA military council were invariably attended by representatives of the Saudi, UAE, Jordanian and Qatari intelligence services, as well as intelligence officers from the US, Britain and France. At one such meeting, apparently in Ankara, Jamal says the Saudi Deputy Defence Minister, Prince Salman bin Sultan, the brother of Saudi intelligence chief Bandar bin Sultan, addressed them all and asked Syrian leaders of the armed opposition “who have plans to attack Assad positions to present their needs for arms, ammo and money”. The impression given is of a movement wholly controlled by Arab and Western intelligence agencies.

The civil war between jihadist groups that started with a co-ordinated attack on Isis positions on 3 January is damaging the standing of all of them. Foreign fighters who came to Syria to fight Assad and the Shia find they are being told to kill Sunni jihadists with exactly the same ideological views as themselves.

The Islamic State sent a suicide bomber who killed Abdullah Muhammad al-Muhaysani, the official al-Qa’ida representative in Syria, and also a leader of Ahrar al-Sham (evidence of how al-Qa’ida has links at different levels to jihadi organisations with which it is not formally associated).

Returning jihadists are finding the way home is not easy, since governments in, for example, Saudi Arabia or Tunisia, which may have welcomed their departure as a way of exporting dangerous fanatics, are now appalled by the idea of battle-hardened Salafists coming back. An activist in Raqqa, seeking to speed the departure of Tunisian volunteers, showed them a video of bikini-clad women on Tunisian beaches and suggested that their puritanical presence was needed back home to prevent such loose practices.

It is a measure of Syria’s descent into apocalyptic violence that the official representative of al-Qa’ida, Jabhat al-Nusra, should be deemed more moderate than Isis. The latter may be on the retreat but this could be tactical and it has a vast territory in eastern Syria and western Iraq into to which to retreat and plan a counter-attack. In any case, Jabhat al-Nusra has always sought mediation with Isis and does not want a fight to the finish. The jihadist civil war has made life easier for the government militarily, since its enemies are busy killing each other, but it does not have the resources to eliminate them.

Crucial to making peace is bringing an end to the proxy war between Saudi Arabia and Iran which is intertwined with the vicious conflict between Shia and Sunni. Russia and the US need to be at one in ending the war, as they briefly seemed to be at the end of last year. Syrians gloomily say the outcome of their civil war is no longer in Syrian hands, but in those of the US, Russia, Saudi Arabia, Iran, Turkey and their various allies.

Peace, if it ever comes, will come in stages and with many false starts such as the failure of the Geneva II peace talks.